As data communication rates are increasing to 25 Gigabits per second (“Gbps”), VCSEL (“Vertical Cavity Surface-Emitting Laser”) apertures are decreasing to limit circuit parasitics, which in turn increase the VCSEL's circuit impedance and forward operating voltage. At 850 nm (“nanometers”) optical wavelengths, the VCSEL forward operating voltage exceeds 2 V (“volts”) and approaches 2.5 V. This increased forward voltage creates problems for the driving electronics, which operate within typical supply voltages of 3.3 V over supply voltage tolerance and operating temperature range. For example, typical transistor turn-on and control voltages are in the range of 0.6 to 0.9 V, and when added to the VCSEL forward voltage approach or exceed the 3.3 V nominal supply at low tolerance.
For example, U.S. Pat. No. 8,767,784 B2 (“Mactaggart”), describes a driver for supplying a modulated current to a laser. As described therein, a control device is configured to enable a laser activation switch to operate as an emitter follower that behaves like a current source to provide current to a VCSEL such that substantially all the current flows from the collector to the emitter of the laser activation switch and substantially no current flows to the base of the laser activation switch. However, the operating point of the control device described in Mactaggart may easily approach the power supply voltage and therefore risk railing the feedback amplifier causing the control device to fail to operate in regulation.
Other than relaxing the system supply requirement and increasing the voltage supply requirement and increasing the voltage supply at proportionally higher power dissipation, a brute-force approach would involve utilizing a boost voltage regulator. Such an approach brings an associated inefficiency in the switching power supply components. Also, transistor breakdown of the driving electronic circuits are decreasing with advanced technology necessary for 25 Gbps operation and therefore may preclude a general increase in the supply voltage to accommodate the increasing VCSEL forward voltage.
With respect to VCSELs, two competing factors are at play. In order for VCSELs to operate faster, the forward voltage for VCSELs is increasing. However, in order for circuits to operate faster in general, the operating voltage of the circuit must be reduced. Another critical consideration is power consumption. Raising the supply voltage for the entire driver circuit may impose a significant power consumption penalty.